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Liaodactylus primus and the ecological evolution of Pterodactyloidea.

Skull of the newfound species Liaodactylus primus (Credit: Chang-Fu Zhou)

Pterosaurs are an extinct monophyletic clade of ornithodiran archosauromorph reptiles from the Late Triassic to Late Cretaceous. The group achieved high levels of morphologic and taxonomic diversity during the Mesozoic, with more than 150 species recognized so far. During their 149 million year history, the evolution of pterosaurs resulted in a variety of eco-morphological adaptations, as evidenced by differences in skull shape, dentition, neck length, tail length and wing span. Pterosaurs have traditionally been divided into two major groups, “rhamphorhynchoids” and “pterodactyloids”. Rhamphorhynchoids are characterized by a long tail, and short neck and metacarpus. Pterodactyloids have a much larger body size range, an elongated neck and metacarpus, and a relatively short tail. Darwinopterus from the early Late Jurassic of China appear to be a transitionary stage that partially fills the morphological gap between rhamphorhynchoids and pterodactyloids.

Pterodactyloidea, the most species-diverse group of pterosaurs, ruled the sky from Late Jurassic to the end of Cretaceous. Liaodactylus primus, a new specimen, discovered in northeast China’s Liaoning province, documents the only pre-Tithonian (145–152 Ma) pterodactyloid known with a complete skull, shedding new light on the origin of the Ctenochasmatidae, a group of exclusive filter feeders, and the timing of the critical transition from fish-catching to filter-feeding, a major ecological shift in the early history of the pterodactyloid clade. The holotype specimen is a nearly complete skull (133 mm long) and mandibles, with the first two cervical vertebrae preserved in articulation with the skull. The elongation of the rostrum, almost half the length of the skull, is accompanied by a significant increase in the number of marginal teeth, giving a total of 152 teeth in both sides of the upper and lower jaws. The teeth are closely spaced to form a ‘comb dentition’, a filter-feeding specialization.

Pterodaustro guinazui cast (Museo Argentino de Ciencias Naturales)

Liaodactylus is the oldest known ctenochasmatid, predating the previously Tithonian (152 Ma) record (Gnathosaurus and Ctenochasma from Germany) by at least 8–10 Myr . The Ctenochasmatidae, represents a long-ranged clade (160–100 Ma), and the only pterodactyloid clade that crossed the Jurassic-Cretaceous transition. The group includes the Early Cretaceous Pterodaustro from Argentina. Popularly called the ‘flamingo pterosaur’, Pterodaustro represents the most remarkable filter-feeding pterosaur known from the fossil record, with a huge number (more than 1000) of densely spaced ‘teeth’ (elastic bristles) in its lower jaws, for filtering small crustaceans, microscopic plankton or algae from open water along lake shores.

Pterosaurs display an extraordinary eco-morphological disparity in feeding adaptations, expressed in skull, jaws and dentition. The Late Triassic Eopterosauria, the basalmost pterosaur clade, were mainly insectivorous. Jurassic insectivores include the Dimorphodontia, Campylognathoididae and Darwinoptera, whereas the Anurognathidae were the only Jurassic insectivores that survived the Jurassic–Cretaceous transition, but became extinct in the Early Cretaceous. The rise of the ctenochasmatid clade was the first major ecological shift in pterosaur evolution from insectivorous-piscivorous to filter-feeding. During Cretaceous time, the Eupterodactyloidea, a group of advanced pterodactyloids, engaged in a variety of feeding adaptations, including filter-feeding, fish-eating, carnivory and scavenging, herbivory including frugivory, durophagy and omnivory. The Early Cretaceous tapejarids may have been herbivorous, while the pteranodontids, with large skull but tapering and toothless jaws were suitable for seizing fish in open-water environments. Finally, the Late Cretaceous azhdarchids have been hypothesized as foragers feeding on small animals and carrion in diverse terrestrial environments.